Comparison of air and liquid for use in loss-of-resistance technique during labor epidurals: A meta-analysis

Background: Air and saline are commonly used in the loss-of-resistance technique to identify the epidural space. However, it is unclear which method promotes more effective analgesic delivery after subsequent epidural catheter placement. Methods: We conducted a meta-analysis to determine the efficacy of air and saline identification methods. We performed a systematic literature search of the National Library of Medicine's PubMed database using terms related to air, saline, epidural, and loss of resistance. Only randomized controlled trials that compared air with saline or local anesthetic were included for analysis. No restrictions were placed on the language of identified articles. Data on pertinent study characteristics and relevant outcomes were extracted from accepted articles. A random effects model was used. Results: The literature search yielded six articles that met all inclusion criteria. A review of the articles reveal 515 subjects for whom air had been used to identify the epidural space and 522 for whom liquid had been used. We were able to obtain pooled estimates for unblocked segments, need for additional medications, and replaced catheters. Use of air was associated with an increased risk for unblocked segments [relative risk (RR) = 2.12, 95% confidence interval (CI): 1.07, 4.21; p = 0.03], but there was no difference with regard to replaced catheters [RR = 0.69, 95% CI: 0.26, 1.82; p = 0.45] or additional medication [RR = 1.59, 95% CI: 0.85, 2.41; p = 0.18]. Conclusion: Our pooled analysis revealed that use of air in the loss-of-resistance technique results in decreased analgesia in one parameter (unblocked segments) but not others (additional medications, replaced catheters). The results should be interpreted with caution, and additional examination with a larger randomized controlled trial is warranted, as the overall number of subjects was relatively small.